Secretion of the type 2 peritrophic matrix protein, peritrophin-15, from the cardia

The midgut of most insects is lined with a peritrophic matrix, which is tho ught to facilitate digestion and protect the midgut digestive epithelial ce lls from abrasive damage and invasion by ingested micro-organisms. The type 2 peritrophic matrix is synthesised by a complex and highly specialised or gan called the cardia typically located at the junction of the cuticle-line d foregut and midgut, Although the complex anatomy of this small organ has been described, virtually nothing is known of the molecular processes that lead to the assembly of the type 2 peritrophic matrix in the cardia. As a s tep towards understanding the synthesis of the peritrophic matrix, the synt hesis and secretion of the intrinsic peritrophic matrix protein, peritrophi n-15 has been followed in the cardia of Lucilia cuprina larvae using immuno -gold localisations. The protein is synthesised by cardia epithelial cells, which have abundant rough endoplasmic reticulum, Golgi, and vesicles indic ative of a general secretory function, Peritrophin-15 is packaged into secr etory vesicles probably produced from Golgi and transported to the cytoplas mic face of the apical plasma membrane. The vesicles fuse with the plasma m embrane at the base of the microvilli and release peritrophin-15 into the i nter-microvilli spaces. The protein then becomes associated with the nascen t peritrophic matrix, which lies along the tips of the epithelial cell micr ovilli. It is proposed that peritrophin-15 binds to the ends of chitin fibr ils present in the nascent peritrophic matrix, thereby protecting the fibri l from the action of exochitinases, (C) 2001 Wiley-Liss, inc.